1. Field of the Invention
The present invention relates generally to a leak detection and location system for chemical storage reservoirs, tanks and conduits containing environmentally harmful materials.
2. Description of the Prior Art
Environmentally harmful materials, such as chemical wastes, are sometimes stored in man-made waste chemical disposal ponds or waste tanks. These containers are generally lined with an impermeable synthetic polymer sheet that prevents the waste chemicals from corroding the tank or contaminating the ground water. Also, pipes and other types of conduits used to transport corrosive chemicals may be similarly lined. One of the problems that has hampered the safe storage and transportation of chemicals in this fashion is that the lining of the pond, tank or conduit can develop leaks and, before the leak can be detected, release harmful materials to pollute the earth and ground water.
In the past, several methods have been devised to detect leaks from chemical waste reservoirs, storage tanks and conduits. For example, bored monitored wells that extend down in the earth to the water table have been used to detect leaks from chemical ponds and subterranean storage tanks. However, this procedure is only suitable for detecting long-term contamination and in some cases there has been substantial contamination of the ground water before the need for corrective action is recognized. Lysimeters have been used as an alternative method of detecting chemical leaks but without much reliability. They tend to clog and break down mechanically and are only capable of extracting test samples in very localized areas.
An alternative method of detecting leaks from waste disposal ponds is disclosed in U.S. Pat. No. 3,252,155, Surtees & Benard, in which the exterior synthetic polymer lining is lined with an electrically conductive metal foil. This method for detecting leaks while the chemicals are stored in the pond requires that the chemicals be electrically conductive. Leaks in the lining are detected by placing one electrical lead on the metal foil and another electrical lead in the chemical stored in the pond, and then connecting these leads to a power source. When a leak occurs in the non-electrically conductive synthetic polymer lining, the electrically conductive waste material contacts the metal foil and completes an electrical circuit that triggers an alarm. When electrically non-conductive materials are stored in the pond, defects in the lining can only be detected when the pond is empty by connecting the metal foil with an electrical lead, placing a second electrical lead in contact with a sponge containing an electrolyte, connecting the two leads to a power source, and tracing the entire liner surface with the sponge. Then when the sponge comes in contact with a defect in the lining, the electrolyte penetrates the lining and completes an electrical circuit, again triggering an alarm.
Another method devised for detecting leaks from subterranean chemical tanks is to place a cured polyester resin sheet on the ground directly beneath the storage tank. The resin sheet is constructed with a drainage system such that leaks from the tank can be detected by inspecting a convenient portion of the drainage system where the fluid will ultimately flow.
A method for detecting leaks from reservoirs containing the waste salt water which is a by-product of petroleum production is disclosed in U.S. Pat. No. 3,383,863, Berry. According to Berry, a salt-water reservoir, e.g., an earthen pit, can be lined with a resin-coated pad of corrugated cardboard, heavy kraft paper or some foamed material. A grid of parallel wires is placed beneath the pad, which grid may be separated by a layer of dry earth from a second, similar grid arranged so that its wires run at cross-angles to the wires of the first grid. Alternatively, the second grid may run within the pad itself. Berry teaches that when a break develops in the pad, the leaking salt water will establish a low-resistance shunt between one or more wires of the first grid and at least one wire of the second grid. As a result, a circuit is completed between the previously isolated grids by the leak, which, according to Berry, can be localized in two dimensions beneath the pad by attempting to pass current through successive pairs of wires in the two grids.
The method for leak detection disclosed by Berry is dependent upon the formation by a leaking electrolytic fluid of a short-circuit across some barrier separating the two grids, at least one of which is in contact with the soil beneath the lined reservoir. Consequently, this approach requires that the soil under the reservoir remain dry, since it is the wetting of the soil by the leaking fluid that completes the circuit, thereby permitting the detection of a change in electrical resistance between the grids. Since soil under reservoirs, tanks and the like is usually moist in most parts of North America for at least part of the year, the method of Berry is not practical in many situations, and therefore has not been utilized commercially to any significant extent.
U.S. Pat. No. 3,564,526, Butts, teaches a technique for detecting leaks in buried pipelines which operates on a principle similar to that disclosed by Berry. In accordance with Butts, a pair of conductors are positioned beneath the buried pipe and separated by an insulation which is degradable by the fluid, such as crude oil, contained in the pipe. When a leak occurs, the insulation is broken down and a short-circuit is established between the two conductors. As in the method of Berry, the leak would be detected according to Butts by passing current through the newly completed circuit.
U.S. Pat. No. 4,107,672, Van Riemsdijk et al, discloses a method for detecting leaks in the heat-insulating lining of a container for cold liquids like liquefied natural gas, whereby a plurality of frangible electrical conductors are incorporated into the heat-insulating lining of woven glass fiber. According to Van Riemsdijk et al, when a crack develops in the lining, one or more of the conductors is broken, triggering an alarm. Van Riemsdijk et al teaches that the crack could then be located within the network of frangible conductors. Thus, the leak detection method disclosed by Van Riemsdijk et al depends on a physical break in the network being caused by the crack's forming in the lining, and does not contemplate a change in the network's electrical properties as a function of contact between the network and liquid leaking from the lined container.
Moreover, the method of Van Riemsdijk et al is intended for use in detecting leaks in liners of rigid containers, and therefore could not be employed advantageously in situations where substantial liner flexibility would be required. For example, the ground beneath earthen storage reservoirs and buried pipelines may shift as much as six to eight inches, upwards or downwards, as the soil alternatively settles or becomes saturated. Consequently, the leak detection method taught by Van Riemsdijk et al, which utilizes a frangible conductive network, could not be applied in practice to chemical storage and transportation that are subject to movement caused by shifting soil.